Use captured step settings when building model context (#44200)

## Why

Model changes during an active turn could leave context using the turn's initial model and settings, even after requests switched to the new model.

## What changed

- Build model, personality, approval, collaboration, app, plugin, and multi-agent instructions from captured step settings.
- Resolve multi-agent policy from the step's effective reasoning effort.
- Use the captured step's context window for full-context metadata and retain that step when rebuilding initial context for compaction.

## Testing

Add regression coverage for model switches updating context without duplicating updates, delegation policy following selected or model-default reasoning effort, and compaction preserving retained context after a model change.

GitOrigin-RevId: 3615c7a3f13f15fdcce9ad13d23311340e0aea88

codex-rs/core/src/compact.rs

@@ -103,10 +103,7 @@ pub(crate) async fn build_compaction_initial_context(
             step_context,
         } => {
             let items = sess
-                .build_initial_context_with_world_state(
-                    step_context.turn.as_ref(),
-                    world_state.as_ref(),
-                )
+                .build_initial_context_with_world_state(step_context, world_state.as_ref())
                 .await;
             (
                 items.into_iter().map(ResponseItemEnvelope::new).collect(),